Color printing processes involve producing full-color images on a medium, such as paper or film, through the use of colorants. Colorants are formed on media by various techniques, including halftone printing, dye diffusion, ink jet, thermal wax, and color laser printing. For example, typical halftone printing processes produce colors as arrays of dots of various colorants. In a typical multi-color, halftone printing technique, an original image is scanned through color filters to form a set of continuous-tone color separations. Each of the color separations represents intensities of one of the separated colors, such as magenta, at a plurality of pixel locations within the original image. The continuous-tone color separations are processed using a halftone screening system to produce a set of halftone color separations in the form of bitmaps. Each of the color separation bitmaps includes a plurality of bits, each bit representing the bi-level condition of one of the separated colors at an addressable unit of the medium. For example, a typical four-color printing process uses four bitmaps, with each addressable unit having four associated bi-level conditions. The addressability of the color separation bitmaps ordinarily is much higher than the addressability of the continuous-tone color separations because several bi-level, addressable units are used to represent the intensity at a single continuous-tone pixel location.
In some applications, color separation bitmaps of this type are used to form halftone printing plates or to control a halftone printing mechanism, such as a thermal mass-transfer device. In either case, the addressable units addressed by the color separation bitmaps are imaged on a printing substrate by formation of device spots carrying colorants that correspond to the separated colors. The device spots are typically sized somewhat larger than the addressable units in order to provide a degree of partial overlap that prevents the appearance of gaps between adjacent spots in areas of solid color. In a typical printing process of this type, the device spots specified by each color separation bitmap are deposited in superposition with one another in substantial registration. The human eye integrates the superimposed colors of the device spots to form a representation of the original continuous-tone image. The formation of differently colored device spots in superposition with one another produces a blending of the colors on the printing substrate. The blending occurs not only between device spots formed in superposition with one another, however, but also between adjacently formed device spots due to the partial overlap caused by the spot sizes. With this blending, for N separated colors, the device spots are capable of forming 2.sup.N different colors within each addressable unit.
Color management systems attempt to produce colors that match across media and devices, locations, or time. Color specification should be device-independent, with all image data having tags that describe how to transform them into a reference color space. These tags may be assigned, for example, by input devices whose profiles describe the transformation from their raw output to the reference color space, or through color pickers within image editing applications. Output devices, such as printers or CRTs, sometimes have associated profiles that describe how to translate reference color space values into the device coordinate values that most closely match the original.
The accuracy of color rendering through typical halftone printing processes is increased by modeling the printer to a device-independent color space. Some types of device-independent color modeling techniques attempt to calibrate devices and data in terms of a device-independent color space, such as Commission Internationale de L'Eclairage (CIE) 1931 XYZ tristimulus space-coordinate values. For example, PostScript and ColorSync provide a fixed framework that defines the processing for converting color specifications from the device-independent color space into an index color space and from the index color space into a device-dependent coordinate system. Device characterization compatible with such frameworks for some classes of printing devices, such as halftone printers, have typically required empirical models that incorporate multi-dimensional lookup tables built from measurements of a large number of printed samples, generally on the order of 500 to 3000, that span the device gamut. For example, one color rendering dictionary (CRD) generation technique requires measurement of a minimum of 512 samples spanning the device gamut (all combinations of each of eight RGB levels). The sample data are typically obtained through time-consuming and tedious manual means or through use of an expensive automated apparatus.
Some printing device models, such as certain models of halftone printers, use the spot diameter of the device as a parameter. It is often assumed that the spot diameter is available either through a priori knowledge or physical measurements. However, some halftone printing devices produce irregular spots or are incapable of producing isolated device spots. These technical limitations make it difficult to measure the spot diameter.